1. Field of the Invention
This invention relates to the field of electromagnetic device design and manufacturing.
2. Description of the Prior Art
An electromagnetic device, such as an electric motor or an electric generator, contains two electromagnetic components: a stationary component known as a "stator," and a rotating component known as a "rotor." In the most common embodiment, the rotor and the stator are cylindrical in shape. The cylindrical rotor is installed inside the hollow, cylindrical stator in such a way that when the rotor rotates, the outer surface of the rotor is proximate to, but does not touch, the inner surface of the stator. The space between the outer surface of the rotor and the inner surface of the stator is known as the "air gap."
It is known in the art that a stator and a rotor each may be manufactured from a core made from a magnetic material, around which or within which insulated electrical conductors known as "windings" are installed. The rotor core and stator core together form the magnetic "flux path" for the electromagnetic device.
A typical stator of a design known in the art is comprised of a hollow, cylindrical core, the inner surface of which contains slots which extend the fall length of the core parallel to the axial direction of the core. The portions of the stator core between the slots are known as the "teeth." The measurement made by adding the width of one slot measured at the base of the slot to the width of one adjacent tooth is known as the "slot pitch."
The prior art stator windings are inserted in the slots in the core, usually by a manual means. After the stator windings are installed into the stator core, the stator may be finished by filling the remaining volume of the stator slots and coating the external surface of the stator with a non-reactive, non-conducting material such as, for example, varnish or epoxy. The non-reactive, non-conducting material serves to protect the stator from corrosion, and to prevent the stator windings from moving within the stator slots during use. Such movement, if permitted, could damage the electrical insulation on the stator windings, and/or could alter the electromagnetic characteristics of the stator.
It is well known in the art to manufacture a stator core from sheet steel. Steel laminations are punched from the sheet steel. The punched steel laminations include slots, alignment holes, and other assembly features. The punched steel laminations are stacked so that the inner surface of the core, the outer surface of the core, the slots, and the alignment holes are aligned. The stacked steel laminations then are secured together by methods known in the art including, for example, welding or riveting.
The method of manufacturing a stator core from sheet steel laminations possesses several disadvantages. The process of punching the steel laminations from the sheet steel creates scrap steel pieces, which often cannot be used productively by the manufacturer. In addition to the cost of the wasted sheet steel pieces, often the manufacturer must incur additional expense involved with the disposal of the wasted sheet steel pieces. Finally, the process of producing the finished stator core from the raw sheet steel is a multiple step process requiring expensive material handling to be performed during and/or between each process step.
U.S. Pat. No. 4,947,065 to Ward et al. disclosed another method for manufacturing a stator core using iron powder particles coated with a thermoplastic material. The method disclosed in U.S. Pat. No. 4,947,065 addresses the disadvantages present in the prior art method of manufacturing a stator core from sheet steel laminations. Scrap is eliminated by the use of a premeasured amount of thermoplastic coated iron particles. The stator core is formed by heating the premeasured amount of the thermoplastic coated iron particles to a predetermined temperature, placing the heated particles into a heated mold that is shaped to produce a stator core of the desired shape, activating a means for compacting the heated particles within the heated mold, thereby compacting the heated particles within the heated mold for a predetermined time at a predetermined pressure. Material handling is reduced because the raw thermoplastic coated iron particle material is manufactured into a finished stator in fewer process steps. The stator core of Ward et al. does not overcome all disadvantages of a prior art stator core made with steel laminations. To fabricate a finished stator from a stator core according to the disclosure of Ward et al., the stator windings must be installed into the slots by a manual means after the stator core is formed, as was required in the stator core made with steel lamination.
Stator windings are conventionally produced from an insulated electrical conductor of types known in the art including, for example, insulated single strand copper wire. The insulated electrical conductor is conventionally formed by methods known in the art into substantially cylindrical winding configurations which will fit within the slots in the stator core, and which will produce the desired electrical effect when the windings are placed in a moving magnetic field, or the desired magnetic effect when the windings are energized with an electric current. The windings are inserted into slots in the stator to maximize the electromagnetic coupling between the windings and the flux path, and to minimize the air gap between the rotor and stator. The portion of the windings which is aligned parallel to the axial direction of the core is conventionally known as the "active portion" of the windings. The portions of the windings which resides outside the stator core at each axial end of the stator core, and which function to conduct electricity from the active portion of the windings which resides in a first slot to the active portion of the windings which resides in a second slot, are conventionally known as the "end turns."
Electric motors and generators operate on the principle of magnetic flux cutting. Electric motors and generators have a source of magnetic flux, such as an electromagnet or a permanent magnet, and a set of windings that intercept the flux. The flux path is always ferromagnetic. The flux is cut when rotation of the rotor occurs. The desired torque and power set the rotor dimension, while the stator dimensions are driven by both the rotor dimension and by the flux return requirements. An important rotor dimension is the "rotor active volume." If "r" is the rotor radius and "l" is the rotor active length, then the rotor active volume "X" is calculated as "X=(.pi.r.sup.2 l)."
A vehicular alternator is an example of electromagnet based electric generator. In a vehicular alternator, the magnetic flux is generated with a multi-pole electromagnet in the rotor. It is desired in the art to maximize the average magnetic flux density, or the "magnetic loading," of the air gap. The magnetic loading may be limited by magnetic saturation of the stator core. A disadvantage present in prior art stator design using internally slotted stator cores, is that the slots reduce the internal surface area of the stator adjacent to the rotating rotor, thereby reducing the ability for magnetic flux to flow between the stator and the rotor. Due to the reduced internal surface area, the stator core teeth reach magnetic saturation more readily than would a stator core without internal slots. When the stator core teeth saturate, the magnetic flux density in the air gap is limited to the ratio of the tooth width to slot pitch multiplied by the saturation flux density of the stator material. For a typical vehicular alternator stator material the saturation flux density is about 1.5 T, and the tooth width to slot pitch ratio is about 1/2, making the magnetic loading about 0.75 T.
Reducing the slot width and increasing the tooth width increases the magnetic loading by increasing the internal surface area of the stator adjacent to the rotating rotor. However, because the slot must carry a fixed total electric current to meet the desired performance characteristics of the electromagnetic device, decreasing slot width requires an increase in slot depth to enable the slot to carry the same total electric current. Increasing the slot depth while maintaining the same total electric current requires increasing the radius of the stator. It is known in the art of motor and generator design to balance magnetic loading and stator dimension in an attempt to find the optimum solution for each application of a particular electromagnetic device.
The magnetic loading of the stator core may be increased without increasing the stator dimensions by fabricating the stator core from a material known in the art to have a higher saturation flux density than steel, such as, for example, an alloy of neodymium iron boron. Such materials improve magnetic loading, but at a substantially higher cost. It is desired to have a stator core fabricated from a readily available, low cost, magnetic material, wherein a higher degree of magnetic loading may be achieved without a corresponding increase in stator size.
Another disadvantage of the prior art stator designs arises from limitations on the amount of electrical current which can be carried in the stator windings installed in a stator slot. The total current carried in the slot is calculated from the current carried in each conductor multiplied by the number of conductors wound into the slot. A typical conductor packing factor for vehicular alternator stators is limited to about 30%, which means that only 30% of the slot volume is occupied by conductors.
It is known in the art that the total current carried in the stator slots, and therefore the conductor packing factor, is limited by the need to dissipate the heat generated by electrical resistance in the conductors. The heat must be dissipated either through the stator core, or through the conductors themselves to the end turns of the conductors. The non-reactive, non-conducting material used to fill the stator slots substantially thermally isolates the conductors from the stator core. As a result, most of the heat must be dissipated through the end turns. The total current carried in the stator slots, and therefore the conductor packing factor, can be significantly increased by providing a direct cooling path through the stator core.
Another disadvantage of the prior art stator designs arises from the significant contribution to stator size made by the end turns of the windings. It is known in the art of vehicular alternator design that end turns add length to a stator while serving no significant power producing function. The end turns are extended to facilitate electric current conduction, and to serve as the heat rejection surface for the windings. The end turns may increase the overall length of a stator by a factor of about 2.5-3 times the active length. It is known in the art that the end turns must be enclosed within the vehicular alternator housing, thus the overall length of the vehicular alternator of this design is increased by similar factor. A stator design wherein the end turns are contained within the volume occupied by the stator core is desirable.
U.S. Pat. No. 5,536,985 to Ward et al. disclosed a rotor assembly wherein the rotor core is comprised of compacted soft magnetic particles coated with a non-magnetic binder, and wherein the rotor windings are embedded within the rotor core. Three methods of manufacturing the rotor assembly are disclosed in U.S. Pat. No. 5,536,985. In the first disclosed method, the rotor core is manufactured by filling a die cavity with a predetermined amount of soft magnetic particles coated with a nonmagnetic binder. The soft magnetic particles within the die cavity are heated at a predetermined temperature and axially compacted for a predetermined time at a predetermined pressure to form the solid rotor core. After the rotor core is removed from the die cavity, preformed discrete rotor windings are embedded into the rotor core in a circular pattern parallel to the axial direction of the core by inserting the discrete rotor windings into preformed holes in the core.
In the second method of manufacturing the rotor assembly disclosed in U.S. Pat. No. 5,536,985, the discrete rotor windings are preformed and inserted into holding devices within a die cavity so that the rotor windings are oriented in a circular pattern parallel to the axial direction of the core. The die cavity then is filled with soft magnetic particles coated with a nonmagnetic binder. The soft magnetic particles and the rotor windings within the die cavity are heated at a predetermined temperature and axially compacted for a predetermined time at a predetermined pressure to form the solid rotor core with the rotor windings embedded therein.
In the third method of manufacturing the rotor assembly disclosed in U.S. Pat. No. 5,536,985, the discrete rotor windings are preformed and inserted into holding devices within a two-part mold comprised of an upper mold part and a lower mold part. The holding devices are included in the lower mold part. The lower mold part containing the rotor windings then is filled with soft magnetic particles coated with a nonmagnetic binder, and the mold is sealed by placing the upper mold part onto the lower mold part. The sealed two-part mold containing the soft magnetic particles and the rotor windings is placed inside a evacuated isostatic compaction chamber. The isostatic compaction chamber is sealed, and the sealed chamber is filled with hydraulic fluid heated to a predetermined temperature, which exerts a predetermined pressure uniformly on all surfaces of the mold. After a predetermined time, the hydraulic fluid is drained from the compaction chamber. The drained chamber is opened and the two-part mold is removed therefrom. The mold is opened destructively to reveal a rotor core with embedded rotor windings.
The performance of an electromagnetic device may be measured by the torque density "D" and power density "Z." These measurements can be derived from the magnetic loading and the electrical loading of the device. The magnetic loading, "B," is the average magnetic flux density of the air gap. The electrical loading "L" is calculated by dividing the total current in the slots "I" by the slot pitch "p." In equation form, it can be represented as "L=(I/p)."
To arrive at the measurements of torque density and power density, several intermediate calculations must be made. First, the tangential force "F" acting on the rotor may be calculated by the equation "F=(BL2.pi.rl)," where "B" is the magnetic loading, "L" is the electrical loading, "r" is the rotor radius, and "l" is the active length of the rotor.
The torque "Y" produced by the rotor may be calculated by the equation "Y=(Fr)," where "F" is the tangential force acting on the rotor, and "r" is the rotor radius. The power "P" produced by the rotor may be calculated may be calculated by the equation "P=(Fv)," where "F" is the tangential force acting on the rotor, and "v" is the rotational speed of the rotor multiplied by the radius of the rotor (known as the "tip speed" of the rotor).
We can normalize the power and torque to the rotor active volume to obtain the torque density and the power density, which are direct measures of performance of the motor or generator. The torque density "D" therefore may be calculated by the equation "D=(Y/(.pi.r.sup.2 l))," where "Y" is torque, "r" is the rotor radius, and "l" is the active length of the rotor. By algebraically substituting for the variables and then algebraically reducing the equation, an equivalent representation of the equation can be shown as "D=(2BL)," where "B" is the magnetic loading, and "L" is the electrical loading.
The power density "Z" may be calculated by the equation "Z=(P/(.pi.r.sup.2 l))," where "P" is power, "r" is the rotor radius, and "l" is the active length of the rotor. By algebraically substituting for the variables and then algebraically reducing the equation, an equivalent representation of the equation can be shown as "Z=((2BLv)/r)," where "B" is the magnetic loading, "L" is the electrical loading, "v" is the tip speed of the rotor, and "r" is the rotor radius.
The measurements of power density and torque density will illustrate to those skilled in the art that magnetic loading and electrical loading are fundamental to electromagnetic device performance. An increase in either the magnetic loading or the electrical loading, or both, relates directly to an increase in power density and torque density. Increasing the tip speed and/or decreasing rotor radius also will increase the power density.
It is desired to produce a stator which overcomes the disadvantages present in the prior art. For example, it is desired to produce a stator which may be manufactured economically from readily available ferromagnetic materials with a minimum of waste and a minimum of material handling, which possesses electromagnetic capabilities required by a variety of applications, and which embodies a smaller, more electrically efficient design.